The importance of magnesium as a plant nutrient has been recognized. Since many cultivated soils are poor in magnesium because of intensive cultivation, it is necessary to eliminate this deficiency by adding to the soil the magnesium-containing fertilizers. Suitable fertilizers for this purpose are mineral multiple-nutrient fertilizers which contain magnesium sulfate in addition to a source of phosphate. Magnesium-containing potash fertilizers or kieserite are also used to supply magnesium.
Potassium magnesium phosphate is a magnesium-containing fertilizer which exhibits excellent plant-physiological efficacy. It can be used either alone or as one of the components of the multiple-nutrient fertilizers. The multiple-nutrient fertilizers with a high total nutrient content are described, for example, in German Pat. Nos. 1,512,210 and 1,592,803.
Potassium magnesium phosphate exists in anhydrous form or as a monohydrate or a hexahydrate salt. Potassium magnesium phosphate hexahydrate has been produced in the prior art by reacting monobasic potassium phosphate with magnesium oxide or magnesium carbonate in the presence of water bound as water of rcystallization. Potassium magnesium phosphate hexahydrate can also be produced by reacting the Engel's salt, MgCO.sub.3.KHCO.sub.3.4H.sub.2 O, with phosphoric acid, as reported in "Gmelin's Handbuch der anorganischen Chemie" [Gmelin's Handbook of Inorganic Chemistry] 8th edition, system No. 27 [B], issue no. 4 (1939), pp. 465-466. The potassium salts used in this process must first be produced from potassium chloride in expensive processes.
According to French Pat. No. 722,378, potassium magnesium phosphate is present in the products produced by kneading calcium dihydrogen phosphate with potassium sulfate and magnesium oxide. Similarly, potassium magnesium phosphate is produced by incorporating potassium sulfate into an acidic basic composition containing calcium phosphate and magnesium phosphate. See, e.g., German Published Application No. 1,767,329. However, the product produced according to this process contains calcium sulfate as an unnecessary inert material.
German Pat. No. 619,397 teaches the production of potassium magnesium phosphate by heating potassium chloride and magnesium chloride with phosphoric acid until the evolution of hydrogen chloride is stopped. However, this process requires the use of expensive apparatus which has high corrosion resistance.
A contaminated potassium magnesium phosphate can be precipitated from seawater with the aid of sodium hydroxide. See, e.g., Japanese Pat. No. 7217.694. Similarly, contaminated potassium magnesium phosphate can be produced by the precipitation of a mixture of ammonium and potassium magnesium phosphate from seawater (Belgian Pat. No. 648,111), or by the precipitation from seawater of other compounds containing potassium magnesium phosphate (British Pat. No. 969,419).
German Unexamined Laid-Open Application, DOS No. 1,924,284 discloses a process for the production of potassium magnesium phosphate wherein magnesium oxide is reacted with phosphoric acid and potassium hydroxide. However, the potassium hydroxide is an expensive starting material because it is produced from potassium chloride in expensive processes.
Another group of the prior art processes for the production of potassium magnesium phosphate is carried out in the presence of organic amines. Thus, for example, German Pat. No. 1,265,726 discloses a process wherein potassium chloride is reacted with magnesium chloride or magnesium sulfate, and with phosphoric acid in the presence of amines to produce potassium magnesium phosphate and corresponding amine salts. The amines can be recovered from the mother liquor by boiling the latter with calcium hydroxide.
U.S. Pat. No. 3,615,186 and Belgian Pat. No. 740,307 disclose analogous processes wherein potassium sulfate is reacted with magnesium sulfate and phosphoric acid in the presence of organic amines to produce potassium magnesium phosphate. Thus, in the presence of the amines, potassium magnesium phosphate can be produced from starting materials which are relatively easily accessible. However, the use of these amines and the necessity of their subsequent recovery results in the additional equipment and capital expenditures which renders such processes expensive and technically complex.